Known multi-layered ceramic circuit boards are typically fabricated by forming layered stacks of ceramic dielectric tape, which are typically known as green tapes or green sheets. The green tapes typically comprise a ceramic material powder and/or glass powder that is mixed with suitable organic binders or resins, solvents, plasticizers and surfactants and formed into a tape. The process for making high density multi-layer circuit boards typically involves forming a plurality of pre-fabricated green tape layers having via holes punched therein, applying or printing circuit patterns on the layers using a conductive ink, i.e., a mixture of a conductive metal powder and a non-metallic powder including a glass and/or a ceramic in a solvent/binder mixture, filling the vias with the conductive ink so that the various circuit layers can be connected to one another through the thickness of the circuit board, and laminating the green tape layers together by pressing the layers into a stack. The stacked layers are then fired at a temperature exceeding 700° C. to burn off organic materials and to densify the green tape material to form a sintered glass and/or ceramic.
The sintered glass and/or ceramic circuit boards are typically quite fragile, however, and in order to impart additional mechanical strength to the laminated board, the circuit boards are often attached to one or both sides of a suitable support substrate, or core material. In addition, it may be desired to mount active, heat generating devices such as integrated circuits onto a high thermal conductivity member made of a metal or certain ceramics (e.g., AlN, SiC, etc.). In such situations, the glass and/or ceramic substrates are typically joined to the support structure via a solder-type bonding procedure, which is performed at low temperatures that are much less than the sintering temperature of the glass and/or ceramic substrate.
It is also possible to join a green laminated structure with a support member or other component before the green laminate is sintered. That is, the green laminated structure is adhered to a support member and the structure is then fired to a temperature that is sufficient to (1) remove the organic materials from the green tapes and the conductive inks, (2) sinter or densify the particles of the green tape composition and the metal particles of the conductive inks to form a sintered multi-layer ceramic and/or glass body, and (3) sufficiently adhere the sintered multi-layer body to the support substrate.
In both co-fired and non-co-fired cases, once bonding and densification have occurred, it is important that the sintered multi-layer body and the support structure have reasonably compatible thermal expansion characteristics, preferably closely matched thermal expansion coefficients. In most cases, however, the thermal expansion coefficient (hereinafter TEC) of the glass and/or ceramic laminate material does not closely match that of the support substrate or core on which the laminates are provided.
This thermal expansion mismatch problem is an important issue with low temperature co-fired ceramics (herein after LTCC) that are common in ceramic packaging applications, where a green laminated structure is sintered at a relatively low temperature after first being adhered to the metal support core. For example, densification of the green laminated structures during sintering can produce a large degree of volume shrinkage in the ceramic and/or glass material, for example, up to about 35–55 percent by volume. If the green laminated body is effectively bonded to an already dense support substrate or core (i.e., a core that does not itself shrink during the sintering process), the green laminated body will be constrained from sintering in the plane of the support substrate/core, which itself does not typically experience densification shrinkage when subjected to the typical sintering temperatures. This can create stresses which will be accentuated by the different expansion behaviors of the glass and/or ceramic substrate and the support member, both during the bonding process and in subsequent use in a thermally active environment.
The difference between the TEC of the material of the multi-layer substrate and that of the support substrate or core can, indeed, lead to substantial problems. For example, significant stresses can develop, particularly at the bonding interface, that lead to warping or other mechanical damage, such as non-adherence to the support substrate and misalignment between vias that are provided in the multi-layered body and the corresponding electrical feed-throughs provided on the support substrate or core. In severe cases, it is possible that the multi-layer substrate will even separate from the support, thus rendering the device unacceptable for use.
Most conventional, commercially available LTCC tapes have a TEC on the order of 6 to 8 ppm/° C. This often does not closely match the TEC of commonly used support cores, including laminated Copper-Molybdenum-Copper support cores, Copper-Molybdenum-Copper support cores that are produced by powder metallurgy techniques, and KOVAR® support cores.
For example, a typical laminated Cu/Mo/Cu (13/74/13) support core has a TEC from room temperature to 300° C. (hereinafter αRT-300) of about 5.3 ppm/° C. and a TEC from room temperature to 600° C. (hereinafter αRT-600) of about 5.6 ppm/° C. When plated with Ni, the laminated Cu/Mo/Cu (13/74/13) support core has an αRT-300 of about 5.75 ppm/° C. and an αRT-600 of about 5.8 ppm/° C. For KOVAR®, the αRT-300 is about 5 ppm/° C.
It is also important that the TEC of the sintered multi-layer glass and/or ceramic substrate closely matches that of the structure to which it is bonded in applications where the multi-layer glass and/or ceramic substrate is bonded to a metal support core after being sintered. A close TEC match is also desirable in stand-alone substrate applications, where the multi-layer glass and/or ceramic substrate is bonded to, or installed in connection with, another member or device, as mentioned above. That is, it is desirable to prevent thermal and mechanical stresses from arising when the final structure is used in the intended applications, which typically include temperature cycling environments.
For the reasons explained above, it would be desirable to provide a material for a multi-layer substrate that has a TEC that closely matches, or that can be easily tailored to match, that of the intended support core or base member to which the multi-layer substrate is to be joined or otherwise installed.
In addition, it would also be desirable to provide a material for a multi-layer substrate that is sufficiently densified at relatively low sintering temperatures, e.g., around 900° C., and that can be suitably provided with, and co-fired with, a relatively low melting point, low resistance metal, such as Ag, Cu and such alloys, especially for LTCC-type applications. Most ceramic and glass materials, however, require significant heat-treatment at temperatures exceeding 900° C. in order to achieve sufficient densification and the desired electrical characteristics. When higher processing temperatures are involved, however, metals having a higher melting point need to be used. High melting point metals, however, have inferior electrical conductivity compared to Ag, Au, and Cu, for example. Thus, it would be more desirable to use a material for the multi-layer substrate that also facilitates the use of such low melting point metals, particularly in co-firing situations.
Most ceramic and glass materials that have a suitable TEC do not also have a suitably low dielectric constant (K) and low loss tangent (or high Q factor), which makes these ceramic materials less desirable for high frequency electronic applications. On the other hand, most ceramic and glass materials having a suitably low dielectric constant and high Q factor do not have a suitable TEC, and in particular, the TEC cannot be tailored to match that of the structure to which the multi-layer substrate is to be bonded.
It would also be desirable, therefore, to provide a multi-layer substrate material that, in addition to having an easily tailorable TEC, also has desirable electrical properties, such as a low dielectric constant (K) and a high Q factor, that is, 1/loss tangent, especially for electronic applications that involve high frequencies (i.e., up to 60 GHz).